The present invention relates to a differential amplifier stage comprising circuit elements for setting the gain to zero and which can be used in amplifier circuits for audio-frequency signals.
Amplifier circuits have at least one input terminal to which is applied a signal of an audio-frequency current or voltage, and at least one output terminal to which is usually connected a loudspeaker, which converts the electrical signal into an acoustic signal.
These amplifier circuits may also include an inhibit terminal with which it is possible to inhibit the amplification, thereby interrupting the production of acoustic signals. When this interruption occurs, transient electrical signals are inevitably produced, which are indistinguishable, for amplification purposes, from the signals applied to the input terminal.
If the spectrum of frequencies of transient signals contains harmonics in the audible range, the loudspeaker converts them into an unwanted, spurious acoustic interference.
The elimination of these transients, or at least the reduction of their harmonic contents is clearly a prerequisite in any circuit operation for carrying out the inhibiting function.
Another prerequisite is that the two enabling and disabling transients for this interruption be as symmetrical as possible in time.
With respect to these prerequisites, a circuit solution of the prior art consists of an amplifier circuit comprising, in addition to the usual signal input terminals, another input terminal, to which a zero signal is applied constantly.
Thus, the inhibit terminal serves to switch the amplifier circuit from the usual input terminals to said other terminal with zero signal with subsequent inhibiting of the acoustic signal.
This solution provides good results in terms of control speed and reduction of the spurious transient signals, but it is not universally applicable precisely because it requires an additional input terminal. In fact, in a monolithically integrated circuit, the number of terminals is fixed by the container selected.
Moreover, there are usually numerous circuit elements that carry out the switching described above, which, at times, necessitates the entire splitting of the input stage of the amplifier circuit.